A liquid crystal display includes a liquid crystal cell and means for illuminating the cell. The liquid crystal cell typically includes a pair of light transmissive substrates with an optically anisotropic liquid crystal material therebetween. The surfaces of each substrate which face the liquid crystal typically have thereon patterned light transmissive electrodes and are also treated, typically by rubbing or by slant evaporation of a dielectric material, to preferentially align the liquid crystal material adjacent to the surfaces.
The properties of the liquid crystal with its long range order are like those of a uniaxial crystal and are described by a director, a unit vector defined as the local average direction of the molecules. The orientation of the director adjacent to a treated surface is typically in the direction of the rubbing or towards the evaporation source. The director may also have a tilt bias, that is, an angle between the director and the substrate surface depending upon the technique used to provide the orientation.
In a twisted nematic cell the substrate and the nematic liquid crystal are typically assembled so that the directors adjacent to the two surfaces are at a non-zero angle, preferably at a right angle, to one another. The liquid crystal between the plates then adjusts so that the director rotates uniformly from the orientation at one surface to that at the other surface. Light polarized parallel or perpendicular to the director at one surface undergoes an optical rotation in passing through the liquid crystal with its plane of polarization being rotated by the twist angle. Plane polarizers oriented parallel to the projection of the director onto each substrate surface will then provide substantially complete transmission of the incident polarized light. Application of a voltage greater than the threshold voltage between the electrodes will cause the director to tilt toward the direction perpendicular to the substrate surfaces, the homeotropic alignment, thereby reducing the optical rotation and causing the light transmission, in the ideal case, to decrease towards zero. The converse of this polarizer arrangement, namely parallel polarizers provides an increase in the transmitted light with applied voltage.
For a small area, essentially personal display the required viewing angle is small. However, in a larger area display, such as a television display, wider viewing angles are desirable in both the vertical and, particularly, in the horizontal plane, relative to the display normal. However, with known techniques the useful range of viewing angles is limited because light rays passing through the cell at different angles experience differing optical birefringence. In particular, the electro-optic curve, which describes the change in the transmitted light as a function of applied voltage, varies with viewing angle. Light transmission through a picture element (pixel) and thus its grey scale then varies with viewing angle. In the extreme case a contrast reversal is observed with varying viewing angle.
One approach to providing this wider viewing angle is to illuminate the cell with highly collimated light in both the vertical and horizontal planes. A two-dimensional diffuser is then positioned between the cell and the viewer to diffuse the transmitted collimated light, as disclosed in U.S. Pat. No. 4,171,874. This approach is undesirable in many applications since the light source must be substantially a point source with its resulting low illuminance or high energy consumption. Alternatively, a line source, such as a fluorescent tube, which provides a point source in one plane and an uncollimated, substantially Lambertian source in the perpendicular plane has been used. This source provides a higher luminance and can be collimated in one plane. The cell typically is illuminated with the light polarized parallel to the director at the light entry surface, collimated in the vertical plane and uncollimated in the horizontal plane. An image displayed on this cell, however, can be viewed only over a limited range of angles in the plane of uncollimated illumination because of the birefringence effects described above.
Thus it would be desirable to have a liquid crystal display having a wide viewing angle, particularly in the horizontal plane which is greater than that presently available.